Clean, dense yttrium oxide coating protecting semiconductor processing apparatus

ABSTRACT

Disclosed herein is a method for applying plasma-resistant coatings for use in semiconductor processing apparatus. The coatings are applied over a substrate which typically comprises an aluminum alloy of the 2000 series or the 5000 through 7000 series. The coating typically comprises an oxide or a fluoride of Y, Sc, La, Ce, Eu, Dy, or the like, or yttrium-aluminum-garnet (YAG). The coating may further comprise about 20 volume % or less of Al 2 O 3 . The coatings are typically applied to a surface of an aluminum alloy substrate or an anodized aluminum alloy substrate using a technique selected from the group consisting of thermal/flame spraying, plasma spraying, sputtering, and chemical vapor deposition (CVD). To provide the desired corrosion resistance, it is necessary to place the coating in compression. This is accomplished by controlling deposition conditions during application of the coating.

The present application is a continuation-in-part application of U.S.application Ser. No. 10/075,967, filed Feb. 14, 2002, now U.S. Pat. No.6,776,873.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to equipment and apparatus used in themanufacture of electronic devices including semiconductor devices, MEMdevices, and other devices used in data manipulation, storage, anddisplay, for example and not by way of limitation. In particular, theinvention pertains to the use of yttrium oxide-comprising protectivelayers which are applied to a surface of apparatus, such as processchamber liners, gas feed plates, substrate support pedestals, andelectrostatic chucks, valves, and similar components which are likely tobe exposed to a corrosive environment. The presence of the protectivelayer protects an underlying material, which makes up a portion of thebody, from the corrosive environment.

2. Description of the Background Art

Corrosion (including erosion) resistance is a critical property forcomponents used in processing chambers where corrosive environments arepresent. This is especially true where high-energy plasma is present andelectrical driving force may be combined with chemical driving force toact upon the surface of components present in the environment.

Process chambers and component apparatus present within processingchambers which are used in the fabrication of electronic devices andMEMS are frequently constructed from aluminum and aluminum alloys.Surfaces of a process chamber and component apparatus present within thechamber are frequently anodized to provide a degree of protection fromthe corrosive environment. However, the integrity of the anodizationlayer may be deteriorated by impurities in the aluminum or aluminumalloy, so that corrosion begins to occur early, shortening the life spanof the protective coating. Ceramic materials of various compositionshave been used in place of the aluminum oxide layer mentioned above, andhave been used over the surface of the anodized layer to improve theprotection of the underlying aluminum-based materials. However, theprotective layer continues to be deteriorated by impurities in thealuminum or aluminum alloy, even though the life span of the protectivelayer is extended. More recently, high purity aluminum alloy materialshave been developed which reduce the tendency of the protective layer orlayers to fail. However, the high purity aluminum alloy materials tendto be expensive.

Yttrium oxide is a ceramic material which has shown considerable promisein the protection of aluminum and aluminum alloy surfaces which areexposed to fluorine-containing plasmas of the kind used in thefabrication of semiconductor devices. In U.S. application Ser. No.10/075,967, the parent application of the present continuation-in partapplication, it is disclosed that a yttrium oxide coating applied overan anodized surface of a high purity aluminum alloy process chambersurface or process component surface produces excellent corrosionprotection. In addition, the evaluation of pure ceramic materials usedas apparatus components has illustrated that a long lifetime for thecomponent may be achieved. However, there remains a need for aprotective layer or coating which can protect the more standard hightemperature aluminum-based materials, such as those in the 2000 seriesand the 5000 through 7000 series.

Japanese Patent Application No. HEI 3[1991]-287797, of Shinji Inazawa etal., published Dec. 18, 1991, discloses a corrosion-resistant ceramicfilm useful in protecting electric wires or component materialsfabricated from aluminum or aluminum alloy from corrosive vapors or lowmelting point metals and highly corrosive inorganic halides and organicmetal compounds. The corrosion-resistant ceramic film is formed by firstproducing an oxidized film formed by the anodic oxidation of the surfaceof aluminum or an aluminum alloy. Subsequently, the oxidized film issubmersed in a solution to provide impregnation with at least one typeof ion selected from the group consisting of chromium ions, yttriumions, zirconium ions, and magnesium ions. The corrosion-resistantprotective film surrounding an electric wire is produced by firing in anoxygen gas stream at 500° C. The particular example illustrating theconcept is a coating on pure aluminum wire, where the ions in theanodized film are created using chromium trioxide aqueous solution.

In a further embodiment of the above-described technology, thecorrosion-resistant material described above is further submersed in asolution of a ceramic precursor consisting of a polymerizable organicmetal compound, followed by heat treatment to form an outer insulationfilm of oxide ceramic.

In U.S. Pat. No. 5,366,585, to Robertson et al., issued Nov. 3, 1994, aplasma processing chamber is described which includes a ceramic barriermaterial, preferably in the range of 130 μm to 250 μm thick, forprotecting metallic walls of the process chamber from attack by theplasma. The ceramic material is said to typically comprise aluminumoxide, although the oxide and fluoride forms of aluminum, magnesium, andtantalum are mentioned. Although free-standing liners are described,protective ceramic layers which are deposited without consuming theunderlying metal (aluminum) substrate are also described. For example,flame-sprayed or plasma-sprayed aluminum oxide is discussed.

U.S. Pat. No. 5,798,016, to Oehrlein et al., issued Aug. 25, 1998,describes a method and apparatus for etching semiconductor devices whereundesirable deposition of films on internal surfaces of the apparatusare prevented using a heatable liner or process chamber wall. The heatedliner or chamber wall may be constructed from a “wide variety ofmaterials, for example, ceramics, aluminum, steel, and/or quartz.Aluminum is the preferred material because it is easy to machine.”However, since aluminum is reactive with a number of plasmas, it isrecommended that “aluminum oxide or a coating thereof be disposed on theliner or chamber walls”, because aluminum oxide tends to be chemicallyinert. In addition to the materials used to construct the liner and/orchamber walls, a protective coating may be applied to the surfaces ofthe liner and/or chamber walls. Examples which are given include Al₂O₃,Sc₂O₃, or Y₂O₃.

U.S. Patent Application Publication No. US 2001/0003271A1, of Otsuki,published Jun. 14, 2001, describes a processing apparatus forsemiconductor wafers, where the process may include a plasma, in which afilm of Al₂O₃, or Al₂O₃ and Y₂O₃, is formed on an inner wall surface ofthe chamber and on those exposed surfaces of the members within thechamber which require a high corrosion resistance and insulatingproperty. An example is given of a processing chamber where a basematerial of the chamber may be a ceramic material (Al₂O₃, SiO₂, AlN,etc.), aluminum, or stainless steel, metal or metal alloy, which has asprayed film over the base material. The sprayed film may contain anoxide of Y, Sc, La, Ce, Eu, Dy, or the like, or fluoride of one of thesemetals. The film may be made of a compound of a III-a element of theperiodic table, such as Y₂O₃ The film may substantially comprise Al₂O₃and Y₂O₃. A sprayed film of yttrium-aluminum-garnet (YAG) is alsomentioned. The sprayed film thickness is said to range from 50 μm to 300μm. There is no description of the manner in which the sprayed film isapplied. There is no description of the condition of the interfacebetween the base material and the film. Further, there is no descriptionof metal impurity concentrations in the sprayed film or loose particleswhich may be present on the film surface. This is important because thecondition of the interface between the base material and the sprayedfilm will have a significant effect on the lifetime of the processchamber. The metal impurity content of the sprayed film and the looseparticles present on the film surface will have a significant effect onthe product yield for product produced in the process chamber, as willbe addressed by applicants during the description of their invention.

U.S. Pat. No. 6,352,611, to Han et al., issued Mar. 5, 2002, describes adielectric window of a reactor chamber where substrates are processed ina plasma of a processing gas. A ceramic composition of matter used toproduce a process kit and a dielectric window preferably contains aceramic compound (e.g., Al₂O₃) and an oxide of a Group IIIB metal (e.g.,Y₂O₃). The ceramic compound may be selected from silicon carbide,silicon nitride, boron carbide, boron nitride, aluminum nitride,aluminum oxide, and mixtures thereof; however, aluminum oxide is said tobe available in a pure form which does not outgas. The Group IIIB metalmay be selected from the group consisting of scandium, yttrium, thecerium subgroup, and the yttrium subgroup; however, yttrium ispreferred, with the oxide being yttrium oxide. The preferred process forforming or producing the dielectric member is by thermal processing of apowdered raw mixture comprising the ceramic compound, the oxide of aGroup IIIB metal, a suitable additive agent, and a suitable binderagent.

U.S. Pat. No. 6,565,984, to Wu et al., issued May 20, 2003, describesthe use of a high purity aluminum alloy to form process chambers andprocessing components used for plasma processing. The high purityaluminum alloy is protected by an anodization layer. By controlling thecomposition of the alloy and the size of particulate inclusions in thealloy, an improved performance is achieved with respect to corrosionresistance for the alloy protected by an anodization layer.

The above-described references are only a few of the backgroundreferences available. However, in view of the existing art known toapplicants, there is still a need for a protective layer or coatingwhich can protect the more standard high temperature aluminum-basedmaterials, such as those in the 2000 series or 5000 through 7000 series,where the alloy composition and size of particulate inclusions can causeproblems of the kind described in the Wu et al. reference. The problemsexist with respect to a protective aluminum oxide coating, whetherformed by anodization or by spray application. Problems similar to thoseoccurring in the aluminum oxide coatings occur when other protectivespray coatings, such as sprayed films containing oxides of Y, Sc, La,Ce, Eu, Dy, or the like, or fluorides of one of these metals, such asY₂O₃ or yttrium-aluminum-garnet (YAG), are applied over a surface of the2000 series or 5000 through 7000 series of aluminum alloys. Thesealuminum alloys are very desirable as substrates for process chamber andcomponent fabrication due to availability and cost, as well asperformance properties not related to plasma corrosion resistance, soability to apply a protective layer with an extended lifetime over thesealloys is important.

SUMMARY OF THE INVENTION

We have determined that it is possible to extend the lifetime of analuminum alloy of the 2000 series or the 5000 through 7000 Series, wherethe aluminum alloy is protected by a plasma-resistant coating containingan oxide of Y, Sc, La, Ce, Eu, Dy, or the like, or a fluoride of one ofthese metals, or yttrium-aluminum-garnet (YAG). An aluminum alloy coatedwith the coating of the invention applied by the method of the inventionshows excellent plasma corrosion-resistance over a lifetime which isextended at least two times, and as much as four times, over thelifetime of a Y₂O₃ coated aluminum alloy of the kind previously known inthe art.

To provide the extended lifetime corrosion resistance described, it isnecessary to place the coating in compression. This is accomplished bycontrolling deposition conditions during application of the coating.Placing the coating under adequate compression helps prevent mobileimpurities in the aluminum alloy substrate from migrating from thesubstrate into the coating and causing defects in the coating whichenable penetration of the coating by reactive species which are incontact with the exterior surface of the coating. Placing the coatingunder compression also increases the density of the coating. Theincreased density of the coating provides better protection fromcorrosive plasmas and improves the machinability of a substrateprotected by the sprayed film. Porosity is an indicator of the densityof the coating, i.e., the less porous the coating, the more dense thecoating. Porosity is expressed as the percentage of open space in thetotal volume of the coating. Yttrium oxide coatings which have beenapplied according to the present method have a porosity of about 1.4%.Typically, yttrium oxide coatings are applied according to the presentmethod under conditions which result in a yttrium oxide film having acompressive stress which is sufficient to provide a yttrium oxide filmporosity of about 1.5% or less. In comparison, yttrium oxide coatingswhich were deposited using prior art methods typically have porositieswithin the range of about 3% to about 5%.

To place the applied coating/film in compression, it is necessary toheat, at least to a nominal depth, the upper surface of the aluminumalloy substrate during application of the coating/film, so that uponcooling of the interfacial surface between the substrate and thecoating, the coating is placed in compression by the contractingaluminum alloy. The upper surface of the aluminum alloy should bepreheated to a depth of at least 250 mils (0.25 inch), and to atemperature of at least about 150-200° C. (Typically, the entiresubstrate to which the coating is to be applied is preheated.) When theupper surface of the aluminum alloy substrate to which the coating isapplied is at a temperature of less than about 150-200° C., the coatingwill not be placed under adequate compression upon cooling to providethe desired corrosion resistance, and will not be sufficiently useful inacting to prevent particulates in the aluminum alloy substrate frommigrating into the coating.

The film/coating may be applied using a number of different methods,such as thermal/flame spray, plasma discharge spray, sputtering, andchemical vapor deposition (CVD). The structure of the coating obtainedis different in each instance. When the coating is applied usingsputtering or CVD, the application rate is much slower, and it may beadvantageous to use the coating in combination with an underlying layerof aluminum oxide. Plasma spray coating has provided excellent results.The protective coating may contain an oxide of Y, Sc, La, Ce, Eu, Dy, orthe like, or a fluoride of one of these metals, oryttrium-aluminum-garnet (YAG). Combinations of the oxides of suchmetals, and/or combinations of the metal oxides with aluminum oxide, maybe used. For example, Y₂O₃ in combination with a minority percentage ofAl₂O₃ (typically, less than about 20% by volume) may be used to improvethermal expansion compatibility of the coating with the underlyingaluminum alloy substrate. This is important when the component ofstructure is exposed to the thermal cycling which often occurs withchemical processing apparatus (such as semiconductor processingapparatus).

The plasma sprayed coating may be applied over a bare aluminum alloysurface. Typically, the aluminum alloy has a very thin film of nativealuminum oxide on its surface, due to exposure of the aluminum surfaceto air. It is advantageous to apply the plasma sprayed coating over thebare aluminum alloy surface, or the surface exhibiting only a nativeoxide, as a better bond between the protective coating comprising anoxide or fluoride of Y, Sc, La, Ce, Eu, Dy, or the like, or YAG isachieved. The plasma sprayed coating may also be applied over analuminum oxide film which is intentionally created upon the aluminumalloy surface. Typically, the thickness of such an oxide coating is lessthan about 4 mils.

When the coated component is to be used in a plasma processing chamberwhere it will be exposed to chlorine species, the plasma sprayed coatingshould be applied over such an intentionally created aluminum oxide filmin order to better protect the underlying aluminum alloy from thecorrosive chlorine plasma. In this instance, the thickness of thealuminum oxide film is typically within the range of about 0.5 mil toabout 4 mils, and the temperature of the aluminum oxide film must be atleast about 150-200° C. at the time of application of the protectivecoating comprising an oxide or fluoride of Y, Sc, La, Ce, Eu, Dy, or thelike, or YAG. The temperature of the aluminum oxide film at the time ofapplication of the protective coating should not exceed the glasstransition temperature of the aluminum oxide.

Typically, the aluminum alloy surface is pre-roughened prior toanodization. The aluminum alloy surface can be pre-roughened using atechnique such as bead blasting or, preferably electrochemical etching,for example, and not by way of limitation.

The applied thickness of the protective coating comprising an oxide orfluoride of Y, Sc, La, Ce, Eu, Dy, or the like, or YAG depends on theenvironment to which the aluminum alloy component or structure will beexposed during use. When the temperature to which the component orstructure is exposed is lower, the thickness of the plasma sprayedcoating can be increased without causing a coefficient of expansionproblem. For example, when the component or structure may be exposed tothermal cycling between about 15° C. and about 120° C., and theprotective coating is yttria (which has been plasma sprayed over analuminum alloy from the 5000 through 7000 series having a native oxidepresent on its surface), the thickness of the yttria coating shouldrange between about 12 mils and about 20 mils. A yttria coating having athickness of about 15 mils provides excellent results. A thinner coatingdown to about 10 mils thickness may be used in combination with ananodized aluminum oxide coating, or other aluminum oxide coating havinga thickness in the 0.5 mil to 4 mils range.

Application of the protective, plasma-resistant coating by plasmaspraying has produced excellent results. When plasma spraying is used,to further improve the performance of the protective, plasma-resistantcoating, it is advantageous to clean the coating after application tothe substrate. The cleaning process removes trace metal impurities whichmay cause problems during semiconductor processing, and also removesloose particles from the surface of the coating which are likely tobecome contaminating particulates during the processing of productadjacent to the coated surface, especially when that product is asemiconductor device.

The cleaning process should remove undesired contaminants and depositionprocess by-products without affecting the performance capability of theprotective coating, and without harming the underlying aluminum alloysurface. To protect the aluminum alloy surface while the coating iscleaned, the coating is first saturated with an inert solvent which doesnot harm the aluminum alloy upon contact. Typically, the coatedsubstrate is immersed in a deionized water ultrasonic bath at afrequency of about 40 kHz (for example, and not by way of limitation)for a period of about 5 minutes to about 30 minutes. Subsequently, achemically active solvent is applied to remove contaminants from theprotective coating. For example, the surface of the coated substrate maybe wiped with a soft wipe which has been wetted with a dilute acidsolution for a period of about 3 minutes to about 15 minutes. Oneadvantageous dilute acid solution comprises about 0.1 to about 5 volume% HF (more typically, about 1 to about 5 volume %); about 1 to about 15volume % HNO₃ (more typically, about 5 to about 15 volume %); and about80 to about 99 volume % deionized water. After wiping, the component isthen rinsed with deionized water, followed by immersion in a deionizedwater ultrasonic bath at a frequency of about 40 kHz (for example, andnot by way of limitation) for a period of about 30 minutes to about 2hours (typically, for a period of about 40 minutes to about 1 hour).

In addition to removing impurities and contaminants from the coatingsurface, the step of wiping the coated component with the dilute HFsolution provides fluorination to the coating surface. Fluorination ofthe coating surface results in a robust, stable coating which is inertto reactive plasmas. Fluorination of the coating surface can also beobtained by exposing the coated surface to a plasma containing fluorinespecies, such as a CF₄ plasma having a density of about 1=10⁹ e⁻/cm³,under conditions which provide a coating surface which is at leastpartially fluorinated.

We have also discovered that application of a protective coatingcomprising an oxide or fluoride of Y, Sc, La, Ce, Eu, Dy, or the like,or YAG, which is under compression in accordance with the presentinvention, permits the drilling of patterns such as attachment openingsthrough the protective coating and underlying aluminum alloy substrate,with less harm to both the protective coating and the interface betweenthe protective coating and the underlying substrate. Laser drilling maybe used to form round through-holes. Ultrasonic drilling providesexcellent results when drilling through-holes of more complicated shapes(such as crescent shapes). The ultrasonic drilling can be carried outeither from the coating surface side or the aluminum alloy substrateside of the component or substrate. The coated substrate may be cleanedafter ultrasonic drilling according to the multiple step cleaningprocess described above.

When ultrasonic drilling a surface of a component or substrate, the bestresults are achieved when a thin sacrificial layer of a flexible,polymeric material is applied or when a more rigid material having asimilar thermal coefficient of linear expansion to that of theprotective coating is applied over the protective coating surface. Thesacrificial layer is typically applied to have a thickness within therange of about 4 mils to about 6 mils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph 100 which illustrates the relative erosion rates 102for various materials upon exposure of the materials to a CHF₃/CF₄plasma

FIG. 2 is a cross-sectional schematic 200 of a type of plasma sprayingsystem which is useful in applying the coatings of the presentinvention.

FIGS. 3A-3C show schematics of photomicrograph views (300, 310, 320) ofthe upper surface of a Y₂O₃ spray coated 6061 aluminum substrate, wherethe spray coating was applied to a thickness of about 200 μm on thesubstrate surface, which was preheated to a temperature of about150-200° C. prior to application of the coating. The view is lookingdirectly down at the substrate, at magnifications of 100×, 500×, and1000× (FIGS. 3A, 3B, and 3C, respectively).

FIGS. 3D-3F show schematics of photomicrograph views (330, 340, 350) ofthe upper surface of the same Y₂O₃ spray coated 6061 aluminum substrateshown in FIGS. 3A-3C. The view of the sample surface was tilted duringimage generation to show the topography in more detail, atmagnifications of 200×, 750×, and 1500×(FIGS. 3D, 3E, and 3F,respectively).

FIGS. 4A-4D show schematics of photomicrograph views (400, 410, 420,430) of a cross-section of a 6061aluminum substrate 402 which has beenspray coated with a layer 404 of Y₂O₃, where the spray coating wasapplied to a thickness of about 200 μm on the substrate surface, whichwas preheated to a temperature of about 150-200° C. prior to applicationof the coating. FIGS. 4A-4C show the cross-section of the spray-coatedsubstrate at magnifications of 100×, 200×, and 250×, respectively. FIG.4D shows the interface between the Y₂O₃ coating 404 and the underlyingaluminum 402 in detail (2000× magnification).

FIG. 5A is a graph 500 illustrating the weight loss of a Y₂O₃ spraycoated 6061 aluminum test coupon (where the spray coating was applied toa thickness of about 200 μm on the substrate surface, and where thesubstrate surface was not preheated prior to application of thecoating), after an ultrasonification treatment was carried out on thetest coupon. The graph shows the weight loss 502 of the coupon as afunction of the time 504 of the ultrasonification treatment.

FIG. 5B is a graph 510 illustrating the weight loss of a Y₂O₃ spraycoated 6061 aluminum test coupon (where the spray coating was applied toa thickness of about 200 μm on the substrate surface, and where thesubstrate surface was preheated to a temperature of about 150-200° C.prior to application of the coating), after an ultrasonificationtreatment was carried out on the test coupon. The graph shows the weightloss 512 of the coupon as a function of the time 514 of theultrasonification treatment.

FIGS. 6A-6D show schematic illustrations (610, 620, 630, 640) ofphotomicrograph cross-sectional views of a hole 600 which has beenultrasonically drilled from the coating side of an aluminum test coupon602 which has been spray coated with a layer 604 of Y₂O₃, where thespray coating was applied to a thickness of about 250 μm, and where thealuminum substrate 602 was preheated to a temperature of 150-200° C. atthe time the coating 604 was applied. FIGS. 6A and 6B showcross-sectional views of the hole 600 at magnifications of 35× and 150×.FIGS. 6C and 6D show cross-sectional views of the left side 606 andright side 608 of hole 600, at a magnification of 250×.

FIGS. 7A-7C show schematic illustrations (700, 710, 720, 730, 740, 750,760) of various photomicrograph top views of the hole 600 shown in FIGS.6A-6D, at magnifications of 25× (FIG. 7A), 50× (FIGS. 7B(1 & 2)), and150× (FIGS. 7C(1-4)).

FIGS. 8A-8D show schematic illustrations (810, 820, 830, 840) ofphotomicrograph cross-sectional views of a hole 800 which has beenultrasonically drilled from the substrate side of an aluminum testcoupon 802 which has been spray coated with a layer 804 of Y₂O₃, wherethe spray coating was applied to a thickness of about 250 μm, and wherethe aluminum substrate 802 was preheated to a temperature of 150-200° C.at the time the coating 804 was applied. FIGS. 8A and 8B showcross-sectional views of the hole 700 at magnifications of 35 × and150×. FIGS. 8C and 8D show cross-sectional views of the left side 806and right side 808 of hole 800, at a magnification of 250×.

FIGS. 9A-9C show schematic illustrations (900, 910, 920, 930, 940, 950,960) of various photomicrograph top views of the hole 800 shown in FIGS.8A-8D, at magnifications of 25× (FIG. 9A), 50× (FIGS. 9B(1 & 2)), and150× (FIGS. 9C(1-4)).

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As a preface to the detailed description, it should be noted that, asused in this specification and the appended claims, the singular forms“a”, “an”, and “the” include plural referents, unless the contextclearly dictates otherwise.

Disclosed herein is a method of applying a plasma-resistant coating onan aluminum or an aluminum alloy substrate. The present method isparticularly useful for applying a plasma-resistant coating to asubstrate which comprises an aluminum alloy of the 2000 series or the5000 through 7000 Series. Because of its greater malleability, 2000series aluminum is particularly preferred when fabricating parts andcomponents having complex shapes.

The plasma-resistant coating comprises an oxide of Y, Sc, La, Ce, Eu,Dy, or the like, or a fluoride of one of these metals, oryttrium-aluminum-garnet (YAG). An aluminum alloy coated with the coatingof the invention applied by the method of the invention shows excellentplasma corrosion-resistance over a lifetime which is extended at leasttwo times, and as much as four times, over the lifetime of a Y₂O₃ coatedaluminum alloy of the kind previously known in the art. Aluminum alloyswhich have been coated with the plasma-resistant coating applied by themethod of the invention are particularly useful as interiors ofsemiconductor processing chambers and as substrates of componentapparatus within such a processing chambers, such as process chamberliners, gas feed plates, substrate support pedestals, and electrostaticchucks, valves, and similar components which are likely to be exposed toa corrosive environment.

To provide the extended lifetime corrosion resistance described, it isnecessary to place the coating in compression. This is accomplished bycontrolling deposition conditions during application of the coating.Placing the coating under adequate compression helps prevent mobileimpurities in the aluminum alloy substrate from migrating from thesubstrate into the coating and causing defects in the coating whichenable penetration of the coating by reactive species which are incontact with the exterior surface of the coating. Placing the coatingunder compression also increases the density of the coating. Theincreased density of the coating provides better protection fromcorrosive plasmas and improves the machinability of a substrateprotected by the sprayed film. Porosity is an indicator of the densityof the coating, i.e., the less porous the coating, the more dense thecoating. Porosity is expressed as the percentage of open space in thetotal volume of the coating. Yttrium oxide coatings which have beenapplied according to the present method have a porosity of about 1.4%.Typically, yttrium oxide coatings are applied according to the presentmethod under conditions which result in a yttrium oxide film having acompressive stress which is sufficient to provide a yttrium oxide filmporosity of about 1.5% or less. In comparison, yttrium oxide coatingswhich were deposited using prior art methods typically have porositieswithin the range of about 3% to about 5%.

To place the applied coating/film in compression, it is necessary toheat, at least to a nominal depth, the upper surface of the aluminumalloy substrate during application of the coating/film, so that uponcooling of the interfacial surface between the substrate and thecoating, the coating is placed in compression by the contractingaluminum alloy. The upper surface of the aluminum alloy should bepreheated to a depth of at least 250 mils (0.25 inch), and to atemperature of at least about 150-200° C. (Typically, the entiresubstrate to which the coating is to be applied is preheated.) When theupper surface of the aluminum alloy substrate to which the coating isapplied is at a temperature of less than about 150-200° C., the coatingwill not be placed under adequate compression upon cooling to providethe desired corrosion resistance, and will not be sufficiently useful inacting to prevent particulates in the aluminum alloy substrate frommigrating into the coating.

The film/coating may be applied using a number of different methods,such as thermal/flame spray, plasma discharge spray, sputtering, andchemical vapor deposition (CVD). The structure of the coating obtainedis different in each instance. When the coating is applied usingsputtering or CVD, the application rate is much slower, and it may beadvantageous to use the coating in combination with an underlying layerof aluminum oxide. Plasma spray coating has provided excellent results.The protective coating may contain an oxide of Y, Sc, La, Ce, Eu, Dy, orthe like, or a fluoride of one of these metals, oryttrium-aluminum-garnet (YAG). Combinations of the oxides of suchmetals, and/or combinations of the metal oxides with aluminum oxide, maybe used. For example, Y₂O₃ in combination with a minority percentage ofAl₂O₃ (typically, less than about 20% by volume) may be used to improvethermal expansion compatibility of the coating with the underlyingaluminum alloy substrate. This is important when the component ofstructure is exposed to the thermal cycling which often occurs withchemical processing apparatus (such as semiconductor processingapparatus).

The plasma sprayed coating may be applied over a bare aluminum alloysurface. Typically, the aluminum alloy has a very thin film of nativealuminum oxide on its surface, due to exposure of the aluminum surfaceto air. It is advantageous to apply the plasma sprayed coating over thebare aluminum alloy surface, or the surface exhibiting only a nativeoxide, as a better bond between the protective coating comprising anoxide or fluoride of Y, Sc, La, Ce, Eu, Dy, or the like, or YAG isachieved. However, the plasma-sprayed coating may also be applied over aceramic surface such as an aluminum oxide film which is intentionallycreated upon the aluminum alloy surface. Typically, the thickness ofsuch an aluminum oxide film is less than about 4 mils.

When the coated component is to be used in a plasma processing chamberwhere it will be exposed to chlorine species, the plasma sprayed coatingshould be applied over such an intentionally created aluminum oxidefilm, in order to better protect the underlying aluminum alloy from thecorrosive chlorine plasma. In this instance, the thickness of thealuminum oxide film is typically within the range of about 0.5 mil toabout 4 mils, and the temperature of the aluminum oxide film must be atleast about 150-200° C. at the time of application of the protectivecoating comprising an oxide or fluoride of Y, Sc, La, Ce, Eu, Dy, or thelike, or YAG. The temperature of the aluminum oxide film at the time ofapplication of the protective coating should not exceed the glasstransition temperature of the aluminum oxide.

Typically, the aluminum alloy surface is pre-roughened prior toanodization. The aluminum alloy surface can be pre-roughened using atechnique such as bead blasting or, preferably electrochemical etching,for example, and not by way of limitation. Bead blasting techniques arewell-known in the art. Commonly owned, copending U.S. application Ser.No. 09/918,683 (“the ′683 application”), filed Jul. 27, 2001, disclosesa method of electrochemically roughening an aluminum or aluminum alloysurface. The electrochemical roughening method disclosed in the ′683application includes the steps of immersing the aluminum-comprisingsurface in an aqueous HCl solution having a concentration ranging fromabout 1 volume % to about 5 volume % (typically ranging from about 1volume % to about 3 volume %), at a temperature ranging from about 45°C. to about 80° C. (typically ranging from about 50° C. to about 70°C.), then applying an electrical charge having a charge density rangingfrom about 80 amps/ft.² to about 250 amps/ft.² (typically ranging fromabout 120 amps/ft.² to about 250 amps/ft.²), for a time period rangingfrom about 4 minutes to about 25 minutes (typically ranging from about 4minutes to about 20 minutes). The HCl solution may further include achelating agent (such as, for example, but without limitation, gluconicacid, available from VWR Scientific Products, West Chester, Pa.), at aconcentration of about 0.5 volume % to about 3 volume %, to control thebath chemistry and conductivity.

FIG. 1 is a graph 100 which illustrates the relative erosion rates 102for various materials upon exposure of the materials to a CHF₃/CF₄plasma. The materials include quartz; polysilicon (“Poly-Si”); singlecrystal silicon; CVD-deposited silicon carbide (“CVD SiC”); anodizedAl₂O₃, having a thickness of 3 mils; spray-coated Al₂O₃, having athickness of 10 mils; bulk Al₂O₃; spray-coated Y₂O₃; and bulk Y₂O₃.

Table One, below, presents process chemistry and conditions which wereused to perform the erosion rate tests in an Applied Materials'PRODUCER™etch chamber (available from Applied Materials, Inc., SantaClara, Calif.).

TABLE ONE Process Conditions for Erosion Rate Test Step: ARC Open MainEtch Overetch CHF₃ (sccm) 200 125 75 CF₄ (sccm) 120 125 0 O₂ (sccm) 1532 0 Ar (sccm) 0 0 200 He coolant applied to 8 8 10 substrate (feed °C.) He pressure * (Torr) 12 12 10 Chamber Pressure (mTorr) 35 50 50Substrate Bias (W) 300 1250 1000 Cathode Temperature (° C.) 15 15 15Wall Temperature (° C.) 15 15 15 Time (hours) 8.7 8.7 2.6 * Pressure ofhelium beneath substrate which is allowed to “leak” around the edges ofthe substrate to provide a flow of cooling fluid over the surface of thesubstrate.

The process conditions for the erosion rate test provided in Table Onewere selected in order to mimic the process conditions to whichsemiconductor processing chamber surfaces are exposed over time duringetch processing of actual semiconductor substrates (such as siliconwafers having various material layers deposited thereon). As shown inFIG. 1, after exposure to the CHF₃/CF₄ plasma, the average erosion rateof plasma spray-coated Y₂O₃ applied by the method of the invention (ave.erosion rate=243 Å/min) is not much different than the average erosionrate of bulk Y₂O₃ (ave. erosion rate=127 Å/min), and is significantlylower than the average erosion rates of conventional coatings such asanodized Al₂O₃ (ave. erosion rate=866 Å/min), spray-coated Al₂O₃ (ave.erosion rate=741 Å/min), and CVD deposited silicon carbide (ave. erosionrate=526 Å/min).

The applied thickness of the protective coating comprising an oxide orfluoride of Y, Sc, La, Ce, Eu, Dy, or the like, or YAG depends on theenvironment to which the aluminum alloy component or structure will beexposed during use. When the temperature to which the component orstructure is exposed is lower, the thickness of the plasma sprayedcoating can be increased without causing a coefficient of expansionproblem. For example, when the component or structure may be exposed tothermal cycling between about 15° C. and about 120° C., and theprotective coating is yttria (which has been plasma sprayed over analuminum alloy from the 5000 through 7000 series having a native oxidepresent on its surface), the thickness of the yttria coating shouldrange between about 12 mils and about 20 mils. A yttria coating having athickness of about 15 mils provides excellent results. A thinner coatingdown to about 10 mils thickness may be used in combination with ananodized aluminum oxide coating or other aluminum oxide coating having athickness ranging from about 0.5 mil to about 4 mils.

Application of the protective, plasma-resistant coating by plasmaspraying has produced excellent results. Plasma spraying is a surfaceprocessing technology in which a powdered material is melted, using thehigh thermal energy of hot plasma, and is blown against the surface of asubstrate material to form a film. The spray material is typically ametal, ceramic, or combination thereof. Plasma spraying has significantadvantages over other types of spray application techniques. Forexample, films having good adhesion to substrate materials can beobtained at fast processing speeds on relatively cold (100° C.-300° C.)substrate materials at atmospheric pressure.

FIG. 2 is a cross-sectional schematic 200 of a type of plasma sprayingsystem (a twin anode alpha torch 238) which is useful in applying thecoatings of the present invention. The particular apparatus illustratedin FIG. 2 is an APS 7000 Series Aeroplasma Spraying System availablefrom Aeroplasma K.K. (Tokyo, Japan). The apparatus 200 includes thefollowing components: first DC main electrode 202; first auxiliaryelectrode 204; first argon source 206; first air source 208; spraymaterial powder source 210; cathode torch 212; accelerator nozzle 214;plasma arc 216; second DC main electrode 218; second auxiliary electrode220; dual anode torches 222A and 222B; second argon source 226; secondair sources (plasma trimming) 228A and 228B; third argon source 236;plasma jet 232; molten powder source 234; and a base material source 224which is to be sprayed.

Twin anode α torch 238 consists of two anode torches, so that each ofthe anode torches bears half of the thermal load. Using twin anode torchα238, a high voltage can be obtained with relatively low current, sothat the thermal load on each of the torches will be low. Each nozzleand electrode rod of the torches is water-cooled separately, and the arcstarting point and ending point are protected by inert gas, so thatstable operation at 200 hours or more is ensured, the service life ofconsumed parts is extended, and maintenance costs are reduced.

A high temperature arc is formed stably between the cathode torch 212and the anode torch 222, and spray material can be fed directly into thearc. The spray material is completely melted by the high temperature arccolumn. The arc starting and ending points are protected by inert gas,so that air or oxygen can be used for the plasma gas introduced throughthe accelerator nozzle 214.

A plasma trimming function 228 is used for twin anode α. Plasma trimmingtrims the heat of the plasma jet that does not contribute to melting ofthe spray material, and reduces the thermal load on the substratematerial and film to making spraying at short distances possible.

One skilled in the art will be able to adapt the method of the inventionto a similar type apparatus used for thermal/plasma spray coating.

When plasma spraying is used to apply the coating, to further improvethe performance of the protective, plasma-resistant coating, it isadvantageous to clean the coating after application to the substrate.The cleaning process removes trace metal impurities which may causeproblems during semiconductor processing, and also removes looseparticles from the surface of the coating which are likely to becomecontaminating particulates during the processing of product adjacent tothe coated surface, especially when that product is a semiconductordevice.

The cleaning process should remove undesired contaminants and depositionprocess by-products without affecting the performance capability of theprotective coating, and without harming the underlying aluminum alloysurface. To protect the aluminum alloy surface while the coating iscleaned, the coating is first saturated with an inert solvent which doesnot harm the aluminum alloy upon contact. Subsequently, a chemicallyactive solvent is applied to remove contaminants from the protectivecoating.

In the coating cleaning process, a coated substrate (such as asemiconductor processing system component) is immersed in a deionizedwater ultrasonic bath (“first DI bath”) at a frequency of about 40 kHz(for example, and not by way of limitation) for a period of about 5minutes to about 30 minutes. This presaturates the coating with waterand removes loose particles from the coating surface prior to chemicalcleaning. The coated component is then chemically cleaned to removetrace metals by wiping the component surface with a soft wipe which hasbeen wetted with a dilute acid solution for a period of about 3 minutesto about 15 minutes. One advantageous dilute acid solution comprisesabout 0.1 to about 5 volume % HF (more typically, about 1 to about 5volume %); about 1 to about 15 volume % HNO₃ (more typically, about 5 toabout 15 volume %); and about 80 to about 99 volume % deionized water.The dilute acid solution should not contact an anodized area of thealuminum alloy surface.

After wiping the component with the dilute acid solution, as describedabove, the component is then rinsed with deionized water, followed byimmersion in a deionized water ultrasonic bath (“second DI bath”) at afrequency of about 40 kHz (for example, and not by way of limitation)for a period of about 30 minutes to about 2 hours (typically, for about40 minutes to about 1 hour). Following removal from the second DI bath,the component is rinsed with deionized water again, then blow dried withN₂ and lamp/oven baked at a temperature of about 50° C. to about 70° C.for a period of up to 2 hours. The component is typically furthercleaned using CO₂ snow (very fine dry ice), which causes any remainingparticles to freeze, crumble, and detach from the component surface.

Table Two, below, presents the results of an analysis of the number of0.2 μm diameter or larger particles per square centimeter remaining on acoated substrate surface after cleaning according to the multiple stepcleaning process described above, with second DI bath times of 10, 40,and 70 minutes.

TABLE TWO Particles Remaining on Coated Substrate Surface After CleaningCleaning Time (minutes) Number of ≧0.2 μm particles/cm² 10 760,000 40240,000 70 230,000

After 40 minutes of immersion in the second DI bath, the number ofparticles present on the substrate surface has been reduced toapproximately one-third of the number of particles remaining after 10minutes immersion in the second DI bath.

Table Three, below, presents the results of an analysis of the surfaceconcentration (×10¹⁰ atoms/cm²) of mobile elements on a coated substratesurface after a standard cleaning process and after cleaning accordingto the multiple step cleaning process described above. The standardcleaning process is a single-step process in which a coated substrate isimmersed in a deionized water ultrasonic bath at a frequency of about 40kHz for a period of up to 1 hour (typically, about 40-50 minutes).

TABLE THREE Surface Concentration of Mobile Elements on Coated SubstrateAfter Standard and Preferred Cleaning Processes Surface Concentration(×10¹⁰ atoms/cm²) Element After Standard Cleaning After PreferredCleaning Chromium (Cr) <20 <20 Copper (Cu) 350 600 Iron (Fe) <20 130Magnesium (Mg) 140,000 17,000 Manganese (Mn) 120 48 Titanium (Ti) <20<20 Zinc (Zn) 2400 120

The surface concentrations of mobile elements magnesium, manganese, andzinc have been greatly reduced after cleaning using the preferredprocess in comparison with the “standard” cleaning process. This reducesthe possibility that these mobile elements will travel to a workpiecesurface during processing in an apparatus protected by the coating.

In addition to removing impurities and contaminants from the coatingsurface, the step of wiping the coated component with the dilute HFsolution provides fluorination to the coating surface. Fluorination ofthe coating surface results in a robust, stable coating which is inertto reactive plasmas. Fluorination of the coating surface can also beobtained by exposing the coated surface to a plasma containing fluorinespecies, such as a CF₄ or CF₄/CHF₃ having a plasma density within therange of about 1×10⁹ e⁻/cm³ to about 1×10⁹ e⁻/cm³, under conditionswhich provide a coating surface which is at least partially fluorinated.

FIGS. 3A-3C show schematics of photomicrograph views (300, 310, 320) ofthe upper surface of a Y₂O₃ spray coated 6061 aluminum substrate, wherethe spray coating was applied to a thickness of about 200 μm on thesubstrate surface, which was preheated to a temperature of about150-200° C. prior to application of the coating. The view is lookingdirectly down at the substrate, at magnifications of 100×, 500×, and1000× (FIGS. 3A, 3B, and 3C, respectively).

FIGS. 3D-3F show schematics of photomicrograph views (330, 340, 350) ofthe upper surface of the same Y₂O₃ spray coated 6061 aluminum substrateshown in FIGS. 3A-3C. The view of the sample surface was tilted duringimage generation to show the topography in more detail, atmagnifications of 200×, 750×, and 1500×(FIGS. 3D, 3E, and 3F,respectively).

The photomicrographs shown in FIGS. 3A-3F show, in detail, themicrostructure of the Y₂O₃ coating on the aluminum substrate. Theroughness of the surface is desirable in that it allows polymer andother byproducts of semiconductor manufacturing processes to collect onsemiconductor processing apparatus surfaces, thereby extending the meantime required between apparatus cleanings.

FIGS. 4A-4D show schematics of photomicrograph views (400, 410, 420,430) of a cross-section of a 6061 aluminum substrate 402 which has beenspray coated with a layer 404 of Y₂O₃, where the spray coating wasapplied to a thickness of about 200 μm on the substrate surface, whichwas preheated to a temperature of about 150-200° C. prior to applicationof the coating. FIGS. 4A-4C show the cross-section of the spray-coatedsubstrate at magnifications of 100×, 200×, and 250×, respectively. FIG.4D shows the interface between the Y₂O₃ coating 404 and the underlyingaluminum 402 in detail (2000× magnification).

The photomicrographs shown in FIGS. 4A-4D illustrate the very densecompression microstructure of the Y₂O₃ coating 404, as well as thesuperior interface between the Y₂O₃ coating 404 and the underlyingaluminum substrate 402.

FIG. 5A is a graph 500 illustrating the weight loss of a Y₂O₃ spraycoated 6061 aluminum test coupon (where the spray coating was applied toa thickness of about 200 μm on the substrate surface, where thesubstrate surface was not preheated prior to application of thecoating), after an ultrasonification treatment was carried out on thetest coupon. The Y₂O₃ coating was not cleaned after application andprior to the ultrasonification. The graph shows the weight loss 502 ofthe coupon as a function of the time 504 of the ultrasonificationtreatment. The weight loss 502 of the coupon increased to about 0.08%within 30 minutes of the start of ultrasonification, then continued toincrease up to about 0.12% after 120 minutes of ultrasonification.

FIG. 5B is a graph 510 illustrating the weight loss of a Y₂O₃ spraycoated 6061 aluminum test coupon (where the spray coating was applied toa thickness of about 200 μm on the substrate surface, where thesubstrate surface was preheated to a temperature of about 150-200° C.prior to application of the coating), after an ultrasonificationtreatment was carried out on the test coupon. The Y₂O₃ coating was notcleaned after application and prior to the ultrasonification. The graphshows the weight loss 512 of the coupon as a function of the time 514 ofthe ultrasonification treatment. The weight loss 512 of the couponincreased steadily to about 0.06% after 120 minutes ofultrasonification. The total weight loss (0.06%) of the coupon withsubstrate preheating was about half of the coupon weight loss (0.12%)without substrate preheating (shown in FIG. 5A).

We have also discovered that application of a protective coatingcomprising an oxide or fluoride of Y, Sc, La, Ce, Eu, Dy, or the like,or YAG, which is under compression in accordance with the presentinvention, permits the drilling of patterns such as attachment openingsthrough the protective coating and underlying aluminum alloy substrate,with less harm to both the protective coating and the interface betweenthe protective coating and the underlying substrate. Laser drilling maybe used to form round through-holes. Ultrasonic drilling providesexcellent results when drilling through-holes of more complicated shapes(such as crescent shapes). The ultrasonic drilling can be carried outeither from the coating surface side or the aluminum alloy substrateside of the component or substrate. The coated substrate may be cleanedafter ultrasonic drilling according to the multiple step cleaningprocess described above.

When ultrasonic drilling from a surface of a component or substrate, thebest results are achieved when a thin sacrificial layer of a flexiblepolymeric material is applied or when a more rigid material having asimilar thermal coefficient of linear expansion to that of theprotective coating is applied over the protective coating surface. Thesacrificial layer is typically applied to have a thickness within therange of about 4 mils to about 6 mils.

FIGS. 6A-6D show schematic illustrations (610, 620, 630, 640) ofphotomicrograph cross-sectional views of a hole 600 which has beenultrasonically drilled from the coating side of an aluminum test coupon602 which has been spray coated with a layer 604 of Y₂O₃, where thespray coating was applied to a thickness of about 250 μm, and where thealuminum substrate 602 was preheated to a temperature of 150-200° C. atthe time the coating 604 was applied. The coated aluminum test coupon602 was cleaned according to the multiple step cleaning processdescribed above after ultrasonic drilling. FIGS. 6A and 6B showcross-sectional views of the hole 600 at magnifications of 35× and 150×.FIGS. 6C and 6D show cross-sectional views of the left side 606 andright side 608 of hole 600, at a magnification of 250×. The edges of thehole 600 showed no evidence of breaking or chipping, and no delaminationof the coating from the aluminum surface was observed.

FIGS. 7A-7C show schematic illustrations (700, 710, 720, 730, 740, 750,760) of various photomicrograph top views of the hole 600 shown in FIGS.6A-6D, at magnifications of 25× (FIG. 7A), 50× (FIGS. 7B(1 & 2)), and150× (FIGS. 7C(1-4)). The bright areas observed in the photomicrographsare optical depth-of-field effects which represent high spots on thecoating surface. Again, the edges of the hole 600 showed no evidence ofbreaking or chipping.

FIGS. 8A-8D show schematic illustrations (810, 820, 830, 840) ofphotomicrograph cross-sectional views of a hole 800 which has beenultrasonically drilled from the substrate side of an aluminum testcoupon 802 which has been spray coated with a layer 804 of Y₂O₃, wherethe spray coating was applied to a thickness of about 250 μm, and wherethe aluminum substrate 802 was preheated to a temperature of 150-200° C.at the time the coating 804 was applied. The coated aluminum test coupon702 was cleaned according to the multiple step cleaning processdescribed above after ultrasonic drilling. FIGS. 8A and 8B showcross-sectional views of the hole 700 at magnifications of 35× and 150×. FIGS. 8C and 8D show cross-sectional views of the left side 806 andright side 808 of hole 800, at a magnification of 250×. The edges of thehole 800 showed no evidence of breaking or chipping, and no delaminationof the coating from the aluminum surface was observed.

FIGS. 9A-9C show schematic illustrations (900, 910, 920, 930, 940, 950,960) of various photomicrograph top views of the hole 800 shown in FIGS.8A-8D, at magnifications of 25× (FIG. 9A), 50× (FIGS. 9B(1 & 2)), and150× (FIGS. 9C(1-4)). Again, the edges of the hole 800 showed noevidence of breaking or chipping

The above described exemplary embodiments are not intended to limit thescope of the present invention, as one skilled in the art can, in viewof the present disclosure expand such embodiments to correspond with thesubject matter of the invention claimed below.

1. A method of providing a plasma-resistant coating on an aluminum oraluminum alloy substrate, where the substrate is a surface of a plasmaprocessing apparatus, wherein said method comprises applying a coatingover at least a portion of a surface of said substrate, wherein saidcoating includes an oxide of an element selected from the groupconsisting of Y, Sc, La, Ce, Eu, Dy, and combinations thereof, whereinsaid coating, which is an oxide of an element selected from said group,is applied to said substrate surface using a technique selected from thegroup consisting of thermal/flame spraying, plasma spraying, andchemical vapor deposition (CVD), and wherein said substrate surface ispreheated to a depth of at least 250 mils to a temperature of at least150° C., and prior to application of said coating over said preheatedsubstrate surface, followed by cooling of said coated substrate to placean exterior surface of said coating in compression, so that a porosityof said coating at said coating exterior surface is less than 1.5%,followed by reaction of said exterior surface of said coating with adilute acid solution comprising about 0.1 volume % to about 5 volume %HF, about 1 volume % to about 15 volume % HNO₃, and about 80 volume % toabout 99 volume % deionized water, to create a fluoride of an elementselected from said group upon said exterior surface of said coating, andwherein said exterior surface of said coating provides an averageerosion rate of said coating when exposed to a fluorine-comprisingplasma of about 243 Å/min or less. [The claim has been amended toclarify that the limitation “substrate” pertains to a surface of aplasma processing apparatus, and not to a substrate (work piece) beingprocessed].
 2. A method in accordance with claim 1, wherein saidfluoride of an element is yttrium fluoride.
 3. A method in accordancewith claim 2, wherein said coating thickness prior to said treatmentwith said dilute acid solution is within the range of about 12 mils toabout 20 mils.
 4. A method in accordance with claim 1, wherein saidsubstrate comprises an aluminum alloy of the 2000 series or the 5000through 7000 series.
 5. A method in accordance with claim 4, whereinsaid substrate comprises an aluminum alloy of the 5000 through 7000series.
 6. A method in accordance with claim 1, wherein said coating isapplied by plasma spraying.
 7. A method in accordance with claim 1,including an additional step in which an aluminum oxide coating isapplied over said aluminum or aluminum alloy substrate prior toapplication of said oxide coating.
 8. A method in accordance with claim7, wherein said aluminum oxide coating over said aluminum or aluminumalloy substrate is heated to a temperature of at least 150° C. andwherein a depth of heating from an upper surface of said oxide coatingand into said substrate is at least 250 mils prior to deposition of saidoxide coating.
 9. A method in accordance with claim 8, wherein saidfluoride of an element is yttrium fluoride.